ARTHROPODA 



273 



continuous with the body only in the region of the head. This 

 mantle is strengthened by five calcareous plates, the cariiM 

 median and dorsal, two scuta near the stalk, and two terga at 

 the free end. Owing to the disposition of these plates, the 

 mantle can only be opened along the ventral surface; when 

 opened the biramous thoracic legs can be protruded, and their 

 movement sets up currents in tlie water which bring food to 

 the mouth. In FoUicipes and Scalpellum, genera in which the 

 stalk is smaller than in Lepas, there are a number of triangular 

 secondary calcareous plates which arise round the top of the 

 stalk and form a kind of collar round the body of the animal. 

 Some of these secondary plates are still further developed in 



Fig. 162.— View of Bed- 

 anus tintinnabulum 

 after removal of the 

 right shell. From 

 Leuokart and Nitsche. 

 After Darwin. 



1. Tergum. 



2. Scutum. 



3. Outer shell in section. 



4. Antennae. 



5. Ovary. 



6. Oviduct. 



7. Opening of oviduct. 



8. Labrum. 



9. Adductor muscle. 



10. Depressor muscle of 

 tergum. 



11. Depressor muscle of 

 scutum. 



12. Mantle cavity. 



Balanus, which is without a stalk, and here they form a 

 calcareous tube consisting of six pieces, the median dorsal one 

 being regarded as equivalent to the carina (Fig. 162). "Within 

 this tube the body of the animal lies protected by four plates, 

 the two scuta and two terga. 



The head is not marked off from the thorax, except by the 

 position of the appendages, and the abdomen is reduced to a 

 short stump, at the end of which the anus is situated. 



Appendages of Lepas. 



1. Antennules. 4. 1st maxillae. 



2. Antennae (lost in larval life).- 5. 2nd „ 



3. Mandible. _6-ll. 6 pairs of thoracic limbs. 



18 



